1. Field of the Invention
The present invention relates to an inspection apparatus and methods of inspection usable, for example, in the manufacture of devices by lithographic techniques.
2. Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., comprising part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning” direction) while synchronously scanning the substrate parallel or anti parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In order to monitor the lithographic process, parameters of the patterned substrate are measured. Parameters may include, for example, the overlay error between successive layers formed in or on the patterned substrate and critical linewidth of developed photosensitive resist. This measurement may be performed on a product substrate and/or on a dedicated metrology target. There are various techniques for making measurements of the microscopic structures formed in lithographic processes, including the use of scanning electron microscopes and various specialized tools. A fast and non-invasive form of specialized inspection tool is a scatterometer in which a beam of radiation is directed onto a target on the surface of the substrate and properties of the scattered or reflected beam are measured. By comparing the properties of the beam before and after it has been reflected or scattered by the substrate, the properties of the substrate can be determined. This can be done, for example, by comparing the reflected beam with data stored in a library of known measurements associated with known substrate properties. Two main types of scatterometer are known. Spectroscopic scatterometers direct a broadband radiation beam onto the substrate and measure the spectrum (intensity as a function of wavelength) of the radiation scattered into a particular narrow angular range. Angularly resolved scatterometers use a monochromatic radiation beam and measure the intensity of the scattered radiation as a function of angle.
Catadioptric optical systems are useful for high numerical aperture objectives in scatterometers because they are compact and allow a wide range of illumination wavelengths. However, reflections at the glass to air interface in catadioptric optical systems used in scatterometers, called ghost reflections, result in unwanted detected signal in the angle-resolved spectrum. Anti-reflective coatings to suppress the ghost reflections are not available that are effective at the wide range of illumination wavelengths over which such scatterometers operate.
Two ghost reflections may be generated at the interfaces, the first order ghost reflection occurs before the illuminating radiation has reached the substrate. The first order ghost reflection is constant in time, has a uniform pupil plane fill, and magnitude of, for example, approximately 2-4% of the detected signal. The second order ghost reflection occurs after the illuminating radiation is reflected from the substrate. Radiation leaving the substrate is reflected back at the air-to-glass interface and via the substrate back into the optical system. The second order ghost reflections are subject to two interactions with the substrate. Therefore, depending on the substrate, the magnitude is much less than for first order ghost reflections, for example, approximately 410−4 to 1610−4 of the detected signal.